Senior UN officials in New York and Geneva are understood to have begun exploring a “last resort” intervention following the refusal of the main Westminster parties to share the pound with an independent Scotland and polls showing rising support for separation north of the border.

Sources within the UN said they were concerned that souring relations between London and Edinburgh in the wake of a victory for SNP leader Alex Salmond could encourage “hotheads” on either side of the border to stoke tensions and seek confrontation.

One scenario being taken seriously is pre-arranged “face-offs”, modelled on the film Braveheart, between rival militias who it is feared could travel to towns such as Gretna Green and Berwick-upon-Tweed for weekend showdowns.